The White/GMC WCS front side marker bulb uses a 1157 size, while the rear side marker bulb uses a 194 size. The replacement chart covers 1988 through 1991 production years with corresponding bulb size specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The White/GMC WCS uses two distinct bulb types across its front and rear side marker positions for the 1988-1991 model years: the 1157 at the front and the 194 at the rear.
The 1157 is a dual-filament bulb rated at 12V with a 21W main filament and a 5W secondary filament, making it capable of serving both active signaling and passive running light functions within a single unit.
The 1157 uses a BAY15d bayonet base, a two-pin offset base design that physically prevents incorrect installation by positioning its two pins at different heights.
The 194 is a miniature wedge-type bulb in the T10 form factor, rated at 12V and 3.78W, intended for low-intensity marker lighting rather than active signaling.
Both the 1157 and 194 conform to ANSI standards applicable in the United States and Canada.
The 1157 and 194 are not interchangeable with each other, as they differ in base type, wattage, filament count, and physical form factor.
The BAY15d base of the 1157 is specific to dual-filament applications, and substituting a single-filament bayonet bulb such as the 1156 would result in loss of one filament function despite a similar base profile.
The T10 wedge format of the 194 is shared by several other low-wattage bulbs, though wattage and voltage ratings must match the original specification for correct operation in the WCS rear side marker position.